1. Field of the Invention
This invention pertains to the generation of radiant energy, more specifically, it describes a new mechanism for tunably generating coherent electromagnetic radiation.
2. Description of the Prior Art
Essentially monochromatic coherent electromagnetic radiation can be generated in several ways, of which probably the best known is by laser action. Another method is to mix coherent radiation in a nonlinear medium, thereby producing sum and difference frequencies, i.e., radiation at frequencies different from that of the input radiation. This disclosure is primarily concerned with this latter process, and I will refer to it as "photon mixing." Since photon mixing is most easily described in a particle picture, I will in the first part of the disclosure talk in terms of photons, their energy, and their momentum. As is well known, photon energy is proportional to frequency .nu., and photon momentum is related to wave number k.
As in all physical processes, in photon mixing both energy and total momentum have to be conserved. In a four-photon process, this means that EQU .nu..sub.1 +.nu..sub.2 =.nu..sub.3 +.nu..sub.4, and EQU k.sub.1 +k.sub.2 =k.sub.3 +k.sub.4 +.DELTA.k
where the subscripts 1 and 2 refer to the initial photons, 3 and 4 to the final ones, and .DELTA.k is the photon momentum exchanged with the mixing medium. For efficient mixing, i.e., for efficient generation of radiation at the desired new frequencies .nu..sub.3 and .nu..sub.4, one needs .DELTA.k=0. In bulk material, this condition can generally not be achieved, since .DELTA.k&gt;0 because of dispersion. Also, in a single-mode solid transmission medium strict conservation is normally not possible. However, K. O. Hill et al., Journal of Applied Physics, 49, page 5098 (1978), have achieved mixing in single mode optical fiber for .nu..sub.1 .apprxeq..nu..sub.2 .apprxeq..nu..sub.3 .apprxeq..nu..sub.4, i.e., very small frequency separations. In the case of a multimode transmission medium, such as multimode optical fibers, strict momentum conservation is possible if the light to be mixed is launched into appropriately selected fiber modes since the wave numbers of higher-order modes are smaller than those of lower-order modes. This fact was used by R. H. Stolen et al., Applied Physics Letters, Vol. 24, page 308 (1974), to achieve four-photon mixing in silica-based fiber optical waveguides. In both these cases the work was done in the visible region, i.e., in a region of normal dispersion of the fibers used.
Recently it has been recognized that, if propagating in a nonlinear medium, a continuous train of electromagnetic waves is subject to a modulational instability. See for instance A. Hasegawa, Plasma Instabilities and Nonlinear Effects, Springer-Verlag, New York, 1975, page 201. It can be shown that this phenomenon is analogous to photon mixing as outlined above, and it is this modulational instability that is exploited by my invention.
One way of using photon mixing is in the generation of monochromatic electromagnetic radiation at frequencies relatively close to the frequency of the radiation source. Another possibility is to use the effect to generate electromagnetic radiation at a relatively lower frequency, by extracting the difference-frequency between, e.g., .nu..sub.3 and .nu..sub.1. The former was practiced, for instance, by K. O. Hill, et al., and by R. H. Stolen, et al., as mentioned above. The latter possibility has typically been used for purposes of frequency measurements only. See, for instance, the review paper by D. J. E. Knight and P. T. Woods, Journal of Physics E, Vol. 9, page 898 (1976).
The invention to be disclosed here can be used to generate coherent radiation at infrared frequencies, in particular, in the far infrared. That spectral region is one in which there still exists a pronounced need for convenient narrow-band tunable sources. Some tunable lasers exist that have their output in the near infrared, such as for instance the double-heterostructure Pb.sub.x Sn.sub.1-x Te diode laser, which has an output range from about 10.mu.m to about 15.mu.m, depending on the temperature of the laser. Another possibility of tunably generating electromagnetic radiation of infrared frequency is parametric oscillation, such as for instance in the LiNbO.sub.3 parametric oscillator source, which has a wavelength range from about 1.4 to about 4 .mu.m. For a review of tunable infrared lasers see for instance the article by A. Mooradian, and for a review of parametric oscillators the article by R. L. Byer, both in the Proceedings of the Loen Conference, Norway, 1976, editors A. Mooradian et al., Springer-Verlag, Berlin, 1976. Thus, tunable sources of coherent radiation exist for the near and middle ranges of the infrared, but they are relatively complex devices. But no convenient such sources appear to exist that have significant output in the far infrared. In view of this situation a simple tunable source of coherent infrared radiation that has a tuning range that includes the far infrared would clearly be of considerable interest.